Specific absorption rate measuring system, and a method thereof

ABSTRACT

A specific absorption rate measuring system, a method thereof, and a biological tissue equivalent phantom unit are disclosed. The biological tissue equivalent phantom unit to be used by the specific absorption rate measuring system for evaluating absorption of electromagnetic wave energy includes a biological tissue equivalent phantom for absorbing an electromagnetic wave; two or more electro-optical crystals that are arranged at two or more measurement points in the biological tissue equivalent phantom, the electro-optical crystals having a dielectric constant that is approximately equal to that of the biological tissue equivalent phantom; and two or more optical fibers laid in the biological tissue equivalent phantom for optically connecting each of the electro-optical crystals to an external destination. The specific absorption rate measuring system for evaluating absorption of the electromagnetic wave energy using the biological tissue equivalent phantom unit includes a luminous source for irradiating a light; a polarization regulator for adjusting a polarization state of the light that is output by the luminous source; an optical-path switcher for providing the polarization-adjusted light to each of the electro-optical crystals one by one; and a specific absorption rate measuring unit for detecting the light reflected from the electro-optical crystal, and for measuring an absorption rate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a biological tissue equivalent phantomunit (phantom unit) used by a specific absorption rate measuring systemfor evaluating absorption of electromagnetic wave energy; a specificabsorption rate measuring system using the phantom unit; and a methodthereof.

2. Description of the Related Art

In recent years and continuing, requirements are increasing forquantitatively evaluating a thermal effect caused by an electromagneticwave emitted by a wireless radio transmitter; and a system that iscapable of accurately and swiftly measuring a specific absorption rate(SAR), which is an index of the reaction of the electromagnetic wave ona living body, such as a human, is desired.

A SAR value is proportional to an electric field (|E|²), and is oftenused for evaluating the energy absorbed by a human body when a cellularphone is used near the human body, SAR being defined by the followingEquation 1.SAR=σ|E| ²/ρ  [Equation 1]

Here, σ and ρ represent conductivity [S/m] and density [kg/m3],respectively, of the biological tissue equivalent phantom.

Usually, when measuring SAR, an electric-field measuring method is used,wherein a short dipole detects an electric field generated in a medium(live body), which electric field is converted into SAR using Equation1.

FIG. 1 shows a conventional specific absorption rate measuring system100 that includes

-   -   a simulated body (phantom) 101 that simulates an electric        constant of a human body with liquid,    -   a container 102 into which the liquid is provided,    -   a probe 103 for detecting an electric field,    -   a probe scanner 104,    -   a signal cable 105,    -   an electric field detection apparatus 106, and    -   a processor apparatus 107 for measurement operations and data        analysis.

Here, the electric field generated in the phantom is measured byarranging a measuring target instrument 108, such as a cellular phone,near the specific absorption rate measuring system 100 as shown inFIG. 1. The probe 103 for detecting the electric field is scanned inthree dimensions by the probe scanner 104, and SAR is measured.

FIG. 2 shows another specific absorption rate measuring system 200 thatincludes

-   -   a phantom 121 that simulates the electric constant of the human        body with a solid-state object,    -   a probe 122 for detecting the electric field,    -   a cable 123 for signal transmissions,    -   an electric-field detection apparatus 124,    -   a processor apparatus 125 for measurement operations and data        analysis, and    -   a scanner 126.

The electric field generated in the phantom is measured by arranging ameasuring target instrument 127, such as a cellular phone, near thespecific absorption rate measuring system 200 as shown in FIG. 2.However, unlike the conventional example shown by FIG. 1, the cellularphone 127 is moved by the scanner 126, and SAR is measured.

In either of the conventional examples, the probe 103 or 122, asapplicable, for detecting the electric field is required. Each of theprobes 103 and 122 for detecting the electric field includes a detectingelement 110 as shown in detail on the right-hand side of FIG. 1. There,an electric field is detected by short dipole elements 111 and 112.Then, the electric field is detected by a Schottky diode 113 inserted ina gap, and a detected result in the form of an electrical signal isprovided to the corresponding electric-field detection apparatuses 106and 124 through high resistance wires 114. That is, the Schottky diode113 detects a voltage generated by the short dipole elements 111, 112formed with conductors, the length of which is about 2 to 5 mm.

However, since the short dipole antenna and the high resistance wires,both being conductive, are present in the electric field to be measured,the electromagnetic field distribution near the detecting element 110 isdisturbed. This is a problem of the electric-field measuring method.Further, since it is difficult to reduce the length of the dipoleelements 111, 112, it is expected that the disturbance will becomegreater as the frequency becomes higher.

Then, in an attempt to reduce the disturbance to the electromagneticfield generated by the measuring target 108, 127 (e.g. a cellularphone), the disturbance being due to the probe 104, 122 for detectingthe electric field, an electric-field sensor 300 using an opticalwaveguide type modulator and a laser beam has been developed as shown inFIG. 3.

The electric-field sensor 300 includes a laser luminous source 131, anelectric-field probe 132, an optical waveguide type modulator 133, aminute dipole 134 that consists of metal, and an optical receiving unit135.

Since the electric-field sensor 300 is configured only by dielectricmaterials, except for the minute dipole 134, it is capable of measuringthe electric field with a precision that is higher than theelectric-field detecting methods that use the high resistance wires.

DESCRIPTION OF THE INVENTION

[Problem(s) to be Solved by the Invention]

Nevertheless, since the short dipole is used according to theelectric-field measuring method using the electric-field sensor 300,wherein the optical waveguide type modulator and the laser beam areused, the disturbance remains, although the disturbance becomes smallerthan in the case of the electric-field measuring methods using the highresistance wires. Further, since the probe for detecting the electricfield, or a 3-dimensional electric-field sensor, is moved in the liquidphantom for measuring SAR, the liquid (a phantom solvent) is agitated,and noise is generated by vibration of the probe or sensor. If a timeuntil the solvent settles into a steady state is waited for in order toavoid the noise, measurement will take a long time. If two or moreelectric-field sensors are arranged in two dimensions or threedimensions in the phantom in order to shorten the measuring time, theaggregate of the sensors (short dipoles) will behave as a conductor, andwill generate a great disturbance to the electromagnetic field to bemeasured. Consequently, a SAR distribution that is different from actualmay be measured, which is a problem.

SUMMARY OF THE INVENTION

The present invention provides a specific absorption rate measuringsystem, a biological tissue equivalent phantom unit, and a methodthereof that substantially obviate one or more of the problems caused bythe limitations and disadvantages of the related art.

Features of the present invention are set forth in the description thatfollows, and in part will become apparent from the description and theaccompanying drawings, or may be learned by practice of the inventionaccording to the teachings provided in the description. Problemsolutions provided by the present invention will be realized andattained by a specific absorption rate measuring system, a biologicaltissue equivalent phantom unit, and a method thereof particularlypointed out in the specification in such full, clear, concise, and exactterms as to enable a person having ordinary skill in the art to practicethe invention.

To achieve these solutions and in accordance with the purpose of theinvention, as embodied and broadly described herein, the inventionprovides a specific absorption rate measuring system, a biologicaltissue equivalent phantom unit, and a method thereof as follows.

[Means for Solving the Problem]

An aspect of the present invention provides a biological tissueequivalent phantom unit that is to be used by a specific absorption ratemeasuring system for evaluating absorption of electromagnetic waveenergy. The biological tissue equivalent phantom unit includes

-   -   a biological tissue equivalent phantom for absorbing an        electromagnetic wave,    -   two or more electro-optical crystals that have a dielectric        constant approximately equal to the dielectric constant of the        biological tissue equivalent phantom, the electro-optical        crystals being arranged at two or more measurement points set up        in the biological tissue equivalent phantom, and    -   two or more optical fibers provided in the biological tissue        equivalent phantom for connecting each of the electro-optical        crystals to an external destination.

According to another aspect of the present invention, a high dielectricconstant material is applied to the surface of the optical fibers of thebiological tissue equivalent phantom unit.

Another aspect of the present invention provides a specific absorptionrate measuring system for evaluating the absorption of theelectromagnetic wave energy using the biological tissue equivalentphantom unit. The specific absorption rate measuring system includes

-   -   a luminous source for emitting a light,    -   a polarization regulator for adjusting a polarization state of        the light emitted by the luminous source,    -   an optical-path switcher for switching the light output by the        polarization regulator to each electro-optical crystal one by        one, and    -   a specific absorption rate measuring unit for measuring a        specific absorption rate by detecting the light reflected by the        electro-optical crystal.

Another aspect of the present invention provides a specific absorptionrate measuring method of evaluating absorption of the electromagneticwave energy using the biological tissue equivalent phantom that receivesirradiation of the electromagnetic wave. The specific absorption ratemeasuring method includes

-   -   a step of arranging two or more electro-optical crystals that        have a dielectric constant approximately equal to that of the        biological tissue equivalent phantom to two or more measuring        points in the biological tissue equivalent phantom,    -   a step of sequentially providing the light to each of the        electro-optical crystals through the optical-path switcher, the        light being irradiated by the luminous source, where the        polarization state of the light is adjusted,    -   a step of reflecting the light that is provided to the        electro-optical crystals,    -   a step of leading the light reflected from the electro-optical        crystal to an analyzer, and    -   a step of converting the light that passes the analyzer into an        electrical signal by a photodetector, and obtaining the specific        absorption rate.

According to another aspect of the present invention, the step ofreflecting the light that is provided to the electro-optical crystals ofthe specific absorption rate measuring method is a step of reflectingthe light by a dielectric reflective film prepared on a surfacecountering a surface, through which the light is provided, of theelectro-optical crystal.

According to another aspect of the present invention, the step ofsequentially providing the light to each of the electro-optical crystalsthrough the optical-path switcher of the specific absorption ratemeasuring method is a step of sequentially providing the light to eachof the electro-optical crystals by selecting an optical fiber by theoptical-path switcher, wherein the optical-path switcher is connected toeach of the electro-optical crystals.

According to another aspect of the present invention, as for thespecific absorption rate measuring method, a high dielectric constantmaterial is applied to the surface of the optical fiber such that theequivalent dielectric constant of the optical fiber becomessubstantially equal to the dielectric constant of the biological tissueequivalent phantom.

As described above, according to the present invention, since theelectric-field detecting element is constituted by nonmetallicmaterials, it is possible to measure the SAR distribution without thedisturbance that is generated in the case of the conventionaltechnology. Further, since the electro-optical crystals having adielectric constant approximately equal to that of the phantom are usedas a sensor head, reflection due to difference of the dielectricconstants is reduced, and the SAR distribution can be more accuratelymeasured. Furthermore, since spatial resolution of the measurement isproportional to a diameter of a beam of the light that penetrates theelectro-optical crystal, the spatial resolution can be raised,theoretically, to as small as the wavelength of the light (several μm).Furthermore, since a change in a refractive index of the electro-opticalcrystal at the measuring point depends on deviation of a dipole thatfollows the electromagnetic wave, the SAR measurement is available in awide band range, from the MHz band to the THz band.

[Effect of the Invention]

According to the present invention, disturbance of the electric field inthe electro-optical crystal by interface reflection is reduced, and theinfluence of the interface reflection on the electromagnetic field nearthe electro-optical crystals is reduced by using the electro-opticalcrystal that has a dielectric constant approximately equal to that ofthe phantom. Therefore, the specific absorption rate measuring systemcapable of obtaining an accurate SAR distribution is realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a conventional specific absorption ratemeasuring system 100;

FIG. 2 a schematic diagram of another conventional specific absorptionrate measuring system 200;

FIG. 3 is a schematic diagram of another conventional specificabsorption rate measuring system 300 using an optical waveguide typemodulator, a laser beam, and an electric-field sensor;

FIG. 4 is a block diagram of a specific absorption rate (SAR) measuringsystem according to an embodiment of the present invention;

FIG. 5 is a perspective diagram showing a phantom according to theembodiment of the present invention; and

FIG. 6 gives a graph showing an error of field strength in anelectro-optical crystal, the error being due to difference in dielectricconstants.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of the present invention are describedwith reference to the accompanying drawings, wherein common referencenumbers are assigned to items that have the same functions, and thedescriptions are not repeated.

FIG. 4 is a block diagram of a specific absorption ratio measuringsystem 40 according to the embodiment of the present invention. Thespecific absorption rate measuring system 40 includes a biologicaltissue equivalent phantom unit 42 that is constituted by a simulatedhuman body (phantom) 1 consisting of liquid, gel, a solid-state object,etc., for simulating the electric constant of the human body,electro-optical crystals 3 that have a dielectric constant approximatelyequal to that of the phantom 1, and bare fibers 10.

The specific absorption rate measuring system 40 further includes

-   -   an electromagnetic wave generator 2 such as a cellular phone,    -   a linearly polarized luminous source 4 such as DFB-LD,    -   a polarization maintenance fiber (PMF) 5,    -   a circulator 6,    -   a polarization regulator 7 that includes a ¼ wave plate and a ½        wave plate,    -   single mode fibers (SMF) 8,    -   an optical-path switcher 9 that is formed by MEMS technology and        PLC technology, and    -   a specific absorption rate measuring unit 44.

The specific absorption rate measuring unit 44 includes an analyzer 11,a photodetector 12, an electrical signal line 13, a signal processingunit 14, and a SAR distribution image display 15.

The specific absorption rate measuring system 40 is for measuring anelectric field in the phantom 1 using the electro-optical crystals 3,the electric field being generated by the electromagnetic wave generator2 (such as a cellular phone) arranged near the phantom 1, as shown inFIG. 4.

The linearly polarized light irradiated by the luminous source 4 isprovided to the polarization regulator 7 via the circulator 6 and thepolarization maintenance fiber (PMF) 5. The polarization regulator 7changes the polarization of the linearly polarized light into apredetermined polarization state, and outputs the light.

The polarization state is determined by a crystallographic axis of theelectro-optical crystal 3 arranged in the phantom 1 and a vibratingdirection of the electric field generated by the electromagnetic wavegenerator 2. For example, when detecting an electric field that vibratesin parallel to the y-axis using CdTe, which is a lead marcasite typecrystal, crystallographic faces (001) (100), and (010) of CdTe areperpendicularly arranged to the y, x, and the z axes, respectively; orto the y, z, and x axes, respectively; and the polarization regulator 7is adjusted so that the polarization axis of the linear polarization oran elliptical polarization may become parallel to the x axis or the zaxis.

The light, the polarization of which has been adjusted, is transmittedvia the single mode fiber (SMF) 8 to the optical-path switcher 9, thento the electro-optical crystals 3.

The light is reflected by a dielectric reflective film prepared on asurface of the electro-optical crystal 3, the surface countering asurface through which the light is provided, and the light goes backalong the incidence path. When the light goes back along the incidencepath within the electro-optical crystal 3, a phase difference arisesbetween components of the predetermined polarization due to arefractive-index change (Pockels effect) that is proportional to thefield strength to the first power that is applied, and the polarizationstate is changed, i.e., polarization modulation occurs.

When CdTe is arranged, e.g., as described above, a phase difference Γarises between polarization components that are parallel to the x axisand the z axis, the phase difference Γ being expressed by the followingEquation 2.Γ=(2π/λ)n ₀ ³ r ₄₁ Ed   [Equation 2]

Here, λ, n₀, r₄₁, E, and d represent the wavelength [m] of the incidentlight, the refractive index of the electro-optical crystal 3, thePockels constant [m/V], field strength [V/m], and the length [m] of theelectro-optical crystal 3 in a direction of the oscillation of theelectric field, respectively.

The light that is reflected and polarization-modulated reaches thecirculator 6 through the optical-path switcher 9 and the polarizationregulator 7; then, the light is branched to the analyzer 11 by thecirculator 6. The modulation component of the branched light is takenout by the analyzer 11, and is converted into an electrical signal bythe photodetector 12. The amplitude of the electrical signal isproportional to the field strength of the electromagnetic wave that ismeasured. The amplitude of the electrical signal is converted into a SARvalue by the signal processing unit 14. Such values, with positioninformation attached, constitute a SAR distribution that can bedisplayed by the SAR distribution image display 15.

According to the electric-field measuring method, the specificabsorption rate (SAR) is defined by Equation 1. According to the presentembodiment, by using the specific absorption rate measuring system 40,SAR can be defined by the following Equation 3, based on Equations 1 and2.SAR=σK|Γ ²|/ρ  [Equation 3]

Here, K is a constant determined by the crystallographic axis of theelectro-optical crystal 3 and the vibrating direction of the electricfield irradiated by the electromagnetic wave generator 2. The constant Kcan be expressed by the following Equation 4 when CdTe is arranged asdescribed above.K=λ/(2πn ₀ ³ r ₄₁ d)  [Equation 4]

Further, when detecting the electric field that vibrates parallel to thex (or z) axis by using CdTe, the crystallographic faces (110), (1(1 bar)0), and (001) of CdTe are arranged perpendicularly to the x(z), y, andz(x) axes, respectively; then the polarization regulator 7 is adjustedso that the polarization axis of the linear polarization or theelliptical polarization may become parallel to the x(z) axis. In thiscase, a phase difference Γ expressed by the following Equation 5 arisesbetween the polarized components that incline 45° to the x axis and thez axis.Γ=(2π/λ)ln ₀ ³ r ₄₁ Ed   [Equation 5]

Here, “l” represents the length [m] of the electro-optical crystal 3 inthe direction of the light passage. Further, when SAR is defined byEquation 3, the constant K can be expressed by the following Equation 6.K=λ/(2πln ₀ ³ r ₄₁)   [Equation 6]

As described above, according to the specific absorption rate measuringsystem 40 of the embodiment, since the electric-field detecting elementis constituted from dielectric materials, the disturbance to theelectromagnetic field to be measured is removed, the disturbanceconventionally being due to the aggregate of the short dipoles, and thedisturbance being the problem with the conventional electric-fieldmeasuring method. The dielectric constants of the phantom 1 areprescribed by ARIB. Although reflection (Fresnel reflection) of theelectromagnetic wave can arise at the interface between the phantom 1and the electro-optical crystal 3 depending on the kind of theelectro-optical crystal 3 according to the difference in the dielectricconstants, such reflection is very small as compared with thedisturbance due to the aggregate of short dipoles.

FIG. 6 shows the field strength in the electro-optical crystal 3 inconsideration of the reflection at the interface when there is noabsorption of the electromagnetic wave within the electro-opticalcrystal 3. In calculation, a model is assumed wherein theelectromagnetic wave is perpendicularly provided to the electro-opticalcrystal 3 that is semi-infinite in size, and as the relativepermittivity of the phantom, a value 40.5 at 1450 MHz that is specifiedby ARIB is used. Calculation results show that a true value can beobtained by compensating for the electric field that is measured byabout 10% for the reflection in the case of CdTe. Further, it isconsidered that the influence on the measured electromagnetic field bythe reflection is proportional to an area ratio that the electro-opticalcrystal 3 occupies. Given that the minimum spatial resolution of the SARmeasurement is 1 mm, and that the minimum processing size of theelectro-optical crystal 3 is about 100 μm, if the reflection factor permm² is converted by the area ratio, it becomes about 1% of 1/100, whichcan practically be disregarded. It is also possible to measure SARwithout compensation if electro-optical crystals that have a dielectricconstant value approximately equal to the dielectric constant value ofthe phantom are used, such electro-optical crystals including LN, LT,and KD*P. The electrical properties of LN, LT, and KD*P and the error inthe measured electric field due to the reflection are shown in Table 1.TABLE 1 Electrical properties and error in measured electric field dueto interface reflection of electro-optical crystals Electro-opticalPockels constant Relative Error in measured crystals (×10⁻¹² m/V)permittivity electric field LiNbO₃ (LN) 19 28 0.8% LiTao₃ (LT) 22 43<0.01% KD₂PO₄ (KD*P) 24.1 48 <0.2%

For the same reason, about 28% of reflection occurs at the bare fiber 10that connects the optical-path switcher 9 and the electro-opticalcrystal 3, which reflection may cause a disturbance to theelectromagnetic field to be measured. The diameter of a common barefiber is 250 μm including a covering layer, and the reflection factorper mm² of the cross section is 1/16 (about 1.8%). The covering layer isprovided in consideration of a micro bend property at low temperatures.However, since the bare fiber 10 of the specific absorption ratemeasuring system 40 according to the embodiment is covered by thephantom 1, a clad fiber having a diameter of 80 μm without a coveringlayer can be used. By using the clad fiber, the reflection factor permm² can be lowered to 0.2% or less.

With reference to FIG. 5, if N electro-optical crystals 3 are arrangedin the direction of the y-axis, the number of the bare fibers 10 on theoptical-path switcher 9 side per mm² is N, and the reflection factor permm² becomes less than 0.2×N %. If the reflection factor is tolerated tobe up to 10%, the number of the electro-optical crystals 3 that can bearranged in the direction of the y-axis becomes 50. If they are arrangedat intervals of 1 mm, the length wherein the electro-optical crystals 3are arranged in the direction of the y-axis is 50 mm. Since the size ofthe phantom that simulates the head is about 300 mm, the reflection bythe optical-path switcher 9 may become great.

On the other hand, it is possible to prevent the reflection of theelectromagnetic wave from occurring by applying a material having agreat dielectric constant to the surface of the bare fibers 10, makingthe equivalent dielectric constant equal to the phantom. Since thedirection of the main axis and sintering temperature can adjust thespecific inductive capacity to a range between 40 and 120, TiO₂ andBaTiO₃ that have a sintering temperature lower than a softeningtemperature (about 1500° C.) of glass are suitable as the material to beapplied.

Further, the present invention is not limited to these embodiments, butvariations and modifications may be made without departing from thescope of the present invention.

The present application is based on Japanese Priority Application No.2004-319387 filed on Nov. 2, 2004 with the Japanese Patent Office, theentire contents of which are hereby incorporated by reference.

1. A biological tissue equivalent phantom unit to be used by a specificabsorption rate measuring system for evaluating absorption ofelectromagnetic wave energy, comprising: a biological tissue equivalentphantom for absorbing the electromagnetic wave; two or moreelectro-optical crystals that are arranged at two or more measurementpoints in the biological tissue equivalent phantom, the electro-opticalcrystals having a dielectric constant approximately equal to thedielectric constant of the biological tissue equivalent phantom; and twoor more optical fibers laid in the live body equivalent phantom forcoupling each of the electro-optical crystals to an externaldestination.
 2. The biological tissue equivalent phantom unit as claimedin claim 1, wherein a high dielectric constant material is applied to asurface of the optical fibers.
 3. The specific absorption rate measuringsystem for evaluating absorption of electromagnetic wave energy usingthe biological tissue equivalent phantom unit as claimed in claim 1,comprising: a luminous source for irradiating a light; a polarizationregulator for adjusting a polarization state of the light; anoptical-path switcher for providing the light output from thepolarization regulator to each of the electro-optical crystals of thebiological tissue equivalent phantom unit one by one; and a specificabsorption rate measuring unit for detecting the light reflected by theelectro-optical crystal, and measuring a specific absorption rate.
 4. Aspecific absorption rate measuring method of evaluating absorption ofelectromagnetic wave energy using a biological tissue equivalent phantomthat receives irradiation of the electromagnetic wave, comprising: astep of arranging two or more electro-optical crystals having adielectric constant that is approximately equal to the dielectricconstant of the biological tissue equivalent phantom to two or moremeasuring points in the biological tissue equivalent phantom; a step ofsequentially providing a light to each of the electro-optical crystalsthrough an optical-path switcher, the light being irradiated by aluminous source, and a polarization state of the light being adjusted; astep of reflecting the light that is provided to the electro-opticalcrystals; a step of leading the light reflected from the electro-opticalcrystals to an analyzer; and a step of converting the light that passesinto the analyzer into an electrical signal by a photodetector, andobtaining a specific absorption rate.
 5. The specific absorption ratemeasuring method as claimed in claim 4, wherein the step of reflectingthe light that is provided to the electro-optical crystals is a step ofreflecting the light that is provided to the electro-optical crystals bya dielectric reflective film prepared on a surface of theelectro-optical crystals, the surface countering a surface through whichthe light is provided to the electro-optical crystals.
 6. The specificabsorption rate measuring method as claimed in claim 5, wherein thelight is sequentially provided to each of the electro-optical crystalsby selecting one of a plurality of optical fibers by the optical-pathswitcher, wherein the optical-path switcher is connected to each of theelectro-optical crystals by a corresponding one of the optical fibers.7. The specific absorption rate measuring method as claimed in claim 6,wherein a high dielectric constant material is applied to a surface ofthe optical fibers.